The present invention relates to an optical element and, more particularly, relates to an optical element suitable for constructing a complex amplitude type spatial optical modulator by assembling a lot of three-dimensional cells and for recording a stereoscopic image as a hologram.
A holographic technique is conventionally known as a method for recording a stereoscopic image on a medium and reconstructing this image. A hologram produced by this method is used in various fields, such as ornamental art or anti-counterfeit seals. In order to optically produce the hologram, it is common to record the interference fringe between object light reflected from an object and reference light on a photosensitive medium. A laser beam superior in coherence is usually used as a light source for the object light and the reference light. Generally, the motion of electromagnetic radiation, such as light, can be regarded as the propagation of a wave front provided with amplitude and a phase, and it can be said that the hologram is an optical element that functions to reconstruct such a wave front. Therefore, it is necessary to record information for accurately reconstructing the amplitude and phase of the object light at each position in space on the recording medium of the hologram. If interference fringes generated by the object light and the reference light are recorded on the photosensitive medium, information that includes both the phase and the amplitude of the object light can be recorded, and, by projecting illumination reconstructing light equivalent to the reference light onto the medium, a part of the illumination reconstructing light can be observed as light provided with a wave front equivalent to the object light.
If the hologram is produced by an optical method using a laser beam or the like in this way, the phase and amplitude of the object light can be recorded only as interference fringes resulting from interference between the object light and the reference light. The reason is that the photosensitive medium has a property of being photosensitized in accordance with light intensity. On the other hand, a technique of producing a hologram by computations with use of a computer has recently been put to practical use. This technique is called a “CGH” (Computer-Generated Hologram) method, in which the wave front of object light is calculated by use of a computer, and its phase and its amplitude are recorded on a physical medium according to a certain method so as to produce a hologram. The employment of this computational holography, of course, enables the recording of an image as interference fringes between object light and reference light, and, in addition, enables the recording of information for the phase and amplitude of the object light directly onto a recording surface without using the reference light.
For example, an optical element, comprising a set of a plurality of three-dimensional cells, is disclosed in U.S. Pat. No. 6,618,190. This optical element functions as a complex amplitude type spatial optical modulator, and by using this art, a hologram can be arranged by the set of three-dimensional cells and a three-dimensional image can be recorded. A specific amplitude and a specific phase are defined in each individual three-dimensional cell of the optical element disclosed in U.S. Pat. No. 6,618,190, and when a predetermined incident light is provided to an individual cell, emitted light, with which the amplitude and phase of the incident light have been changed in accordance with the specific amplitude and specific phase defined in the cell, is obtained. That is, each individual cell has unique optical characteristics and functions as an element (complex amplitude type spatial optical modulation element) in which a specific amplitude and a specific phase are recorded.
As specific examples of three-dimensional cells having the function of recording both amplitude and phase, the above-mentioned U.S. Pat. No. 6,618,190 discloses cells, each having a groove formed by hollowing a portion, of an area that is in accordance with a specific amplitude, by just a depth that is in accordance with a specific phase, and cells, each having a convex part formed by protruding a portion, of an area that is in accordance with a specific amplitude, by just a height that is in accordance with a specific phase. An optical element comprising a set of three-dimensional cells having such characteristic shapes can be manufactured by a manufacturing process using an electron beam drawing device, etc., and thus has the merit of being suited for mass production.
However, with the optical elements using three dimensional cells that are disclosed as embodiments in the above-mentioned U.S. Pat. No. 6,618,190, noise components become mixed in during reconstruction and clear reconstruction results thus cannot be obtained necessarily. This is because a part of the incident light that is provided as illumination light in the reconstruction process or a part of the reflected light of this incident light is observed as 0th-order diffraction light. In particular, when such an optical element that gives rise to such 0th-order diffraction light is used in combination with a lens, the 0th-order diffraction light becomes converged at the focal point position of the lens and cannot be neglected in terms of practical use.